21

Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 27/03/2020 16:14:54 »

With D moving at 0.5c towards the Earth and the Earth receding at 0.2c, the closing speed between D and the Earth, according to B is 0.3c,  at which rate, it will take 20 years for D to reach the Earth.

That doesn't seem like relativistic velocity addition.
To make the scenario perfectly symmetrical using those numbers, in B frame D should aim to planet Y which is 10 lightyears away. At speed of 0.5c, it will take 20 years to reach planet Y.

As point out by Halc, it isn't needed here.
Example, You have Alice, Bob, and Charlie.
Alice "stays home", while as measured by her, Bob and Charlie both head off in opposite directions at 0.5c
According to Alice,  With Bob going in one direction and Charlie in the other, the distance between them is increasing at a rate of 0.5c + 0.5c = 1c . After 1 hr they will be 1 light hr apart from each other ( Bob will be 0.5 light hr from Alice in one direction, and Charlie 0.5 light hr from Alice in the the Other.) etc.
For Bob to get the relative speed between himself and Charlie, he adds the 0.5c he measures relative to Alice, to the 0.5c Alice measures as the relative velocity between herself and Charlie, using relativistic velocity addition.
(.05c+..5c)/1(1+0.5c(0.5c)/c^2) = 0.8c
The difference between Charlie's velocity and Alice's velocity as measured by Bob is 0.8c-0.5c = 0.3c
After 1 hr ( by Bob's clock) Bob measures Alice to be 0.5 light hr away, and Charlie as being 0.8 light hr way. A difference of 0.3 light hr.

22

Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 26/03/2020 15:46:38 »

Let's make the situation more symmetrical by adding planet Y, which is stationary in B frame and located 10 light years behind, measured in B frame. How far is the distance between B and planet Y when measured by earth observer?
Let's say the earth observer also has a twin, called D. When B is passing by and A jump to earth, D jump into B's space ship. After 30 years in the ship, D return to earth at 0.5c measured in B frame. Will he arrive on earth at the same time as planet Y passing by earth?

From the Earth frame:
Y is 9.798 ly away as B passes, and is approaching at 0.2c.  It will take 48.99 yrs to reach Earth.
D has jumped on to B's ship, an stays for 30 yrs (B's time)., This takes 30.62 years by Earth's clock.  This means that at that point B and D will be 0.2c x 30.62 = 6.124 ly from Earth.
D heads back towards Earth at 0.5c relative to B as measured by B,  this equates to (0.2c-0.5c)/(1-(0.2c)(0.5c)) = -0.333...c
(minus sign represents direction back towards Earth.)
6.124 ly/ 0.333c = 18.372 y
Added to the 30.62 yrs equals 48.99 yrs.  The same amount of time that passed between B passing and Planet Y arriving.

In B's frame:
Y is 10 light years away when he passes Earth and D jumps aboard. After 30 yrs of flight, B and D are 6 ly from Earth. And Y is 4 ly from the Earth.  With D moving at 0.5c towards the Earth and the Earth receding at 0.2c, the closing speed between D and the Earth, according to B is 0.3c,  at which rate, it will take 20 years for D to reach the Earth. It also will take 20 yrs for planet Y to cover 4 ly at 0.2c.  So according to B, it takes 50 yrs between his passing Earth and both D and Y to meet up at the Earth.   Meanwhile, according to B, the clock on Earth has been time dilated and accumulated 48.99 yrs. The exact same amount of time as determined by the Earth.

23

Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 24/03/2020 19:41:38 »

Quote from: hamdani yusuf on Yesterday at 08:47:43
Here is the same scenario, but the twins start from earth simultaneously.
And here is the same scenario, but the twins finish on planet X simultaneously.

We can see that in B's frame, B doesn't change velocity since beginning of the trip until the end, although the diagrams don't show B's position before the journey start and after it ends.
Would it make a difference if B never changes velocity at all?
Let's say that the twin were born in a space ship moving at 0.2c on a line connecting earth and Planet X. When the ship is passing earth, twin A stops by on earth for 30 years in earth time, and then catching up B with faster space ship at 0.5c relative to earth, which makes them meets again after 20 years journey from earth to planet X (measured in earth time). The question is, what's the symmetry breaker between B and earth observer, giving that they never really change their frame of reference?

If B never changes velocity, then the symmetry between B and the Earth observer is never broken. 

Space-time diagram from the Earth observer frame.
 
Image1.gif (6.81 kB . 360x566 - viewed 5546 times)

Dark blue line - Earth
Green line - distant planet
blue line - B
Red line - A (if he leaves Earth at the same time as B)
magenta line -A ( if he waits at Earth and then leaves)
Times for each clock shown.  It is assumed that in the Earth rest frame clocks at the distant planet are synchronized to the Earth clock.
The two times separated by the / represents Distant planet time/ A's time

If you shift to B's frame of reference with no changes in velocity, you get this:


Image2.gif (7.43 kB . 277x562 - viewed 5491 times)
When B passes Earth the Earth clock reads zero, B sets his clock to zero also (or B is  born, if you like.)  At this moment the clock at the distant planet already reads 2 years past 0.
B takes ~ 49 years by his clock to reach the distant planet, during which time, Both the Earth clock and distant planet clock advances ~48 years.  Thus the planet's clock reads 2+48 = 50 years upon his arrival, while according to him the Earth clock reads 0+48 = 48 years at that same moment.

The cyan line represents the "checkpoint" that you mentioned earlier and where B would be at when A(red line ) reaches the planet (as measured in the Earth-planet frame.)  In B's frame he doesn't reach this point until after A has reached the distant planet.

The point is that if B never changes velocity with respect to the Earth, B and and the Earth will disagree as to who aged more between B passing Earth and passing the planet.

24

Physics, Astronomy & Cosmology / Re: What would I find in a square centimetre of space?

« on: 07/03/2020 15:58:28 »

As the title says, just what would I find in a square centimetre of space? If that bit of space was in the vacuum of outer space?

It depends on where in "outer space" you are.  The interstellar medium can vary from a density of 1e-4 atoms per cm^3 to 10^6 atoms per cm^3. While 1 million atoms may seem like a lot, it is still 1/10,000 as many as you would find per cubic centimeter in a man-made high-vacuum chamber.

25

Physics, Astronomy & Cosmology / Re: What Do You Call A Moon Of A Moon ?

« on: 14/12/2019 16:27:36 »

One  specific answer is that we call it the Moon.
The Earth is a "moon" of the Sun.

Thank Ewe BC. So, our "moon" is called "The Moon" is that it's name ? or does our Moon not have one ?

Does this then mean that the Sun is a moon of Sagittarius A ?

To be accurate, there is only one "Moon". It is the natural satellite of the Earth that takes roughly a month to complete one synodic period (the word moon comes from the word meaning "month".

The tendency to call other natural satellites "moons" is similar to calling all open-end adjustable wrenches "Crescent" wrenches.  Crescent is the brand name, and only the wrenches made by that company are truly "Crescent wrenches".  The company name just got closely associated with that type of wrench.

26

Physics, Astronomy & Cosmology / Re: Is everything in the universe in motion?

« on: 12/12/2019 21:23:23 »

It is said that our solar system is orbiting the galaxy and the galaxies are orbiting each other.. When we look out into space it seems as if everything is stationary but that is clearly not the case.

The Solar system has a motion relative to the center of the galaxy. The galaxy has a motion relative to the barycenter of the local group.     If we look out at a distant galaxy, it has a motion relative to us.   The key word in all of these statements is "relative".   Motion can only be measured with respect to a reference, and that reference can be arbitrary.  When we look at that distant galaxy, and claim that it moves relative to our galaxy, we could just as easily claim that is our galaxy moving relative to it.   We generally say the former because it is the most convenient.   Thus is the case for any reference frame, we simply choose the one that is convenient for our needs.    There is no absolute reference against which we can measure motion.

27

Physics, Astronomy & Cosmology / Re: A reported object travelling faster than the speed of light?

« on: 26/11/2019 18:13:31 »

 A key is understanding the difference between "apparently" moving faster than light and actually moving faster than light.
Simple example:  And object starts 8 light mins away traveling towards you at 0.8c.
The light from the moment it starts at the distance of 8 light min, arrives 8 min after it starts.  The object itself arrives in 10 min, or only 2 min after the light of it being 8 light min away does.  Thus it would appear to you that the object crossed the 8 light min distance in just 2 min and thus traveled at 4c.   But this is just an visual effect caused by the fact that the object is following closely behind its own light. The object was always traveling at 0.8c You merely saw its 10 min trip compressed into 2 min.

28

New Theories / Re: Does Einstein’s Equivalence Principle have any physical standing?

« on: 17/11/2019 16:54:02 »

Let’s just suppose (as a special case) that in interstellar space there is a planet exactly like earth about 50 billion miles away from the space ship, which weighs 2 tons or 2000 Kg. Then the acceleration due to that  planet would be 9.8m/s2 and the gravitational  attraction between the planet and the space ship would be  G x (5.972 x 1024 x 2x 103)/((5X1010)2) = 0.000318860846 m3 kg-1 s-2 .

For one, you screwed up your equation.  You put the (2 x 10^3), which I assume was a rough attempt to convert miles to meters*,  in the wrong half of the equation   It needed to be multiplied by the 5 x 10 so that the you had
 G x (5.972 x 1024 )/((5X1010x 2x 103)2) . In other words, the numerical value of your number was off by a factor of at ~ 8e10. in addition to being expressed in the wrong units.  Correctly used, the equation gives an answer in m/s^2 or acceleration.  For example, the equation used for the surface of the Earth (a distance of ~6378000 meters from the Earth center gives
G(5.972e24/6378000^2 = 9.8 m/s^2 or the acceleration due to gravity at the surface.)
A more accurate value for your scenario is 6.23e-14 m/s^2.    Which is the acceleration due to gravity at this distance from the Earth. 

It is now possible to see that the gravity exerted on the space ship is very small, therefore if the mass of the spaceship were to be multiplied by the force of gravity a total force of 0.00031 N which is an extremely small force would be exerted by gravity. If the ship were being accelerated at 9.8m/s2 then the force exerted on the space ship would be equal to F = 2000 Kg x 0.00003 m/s1 = 0.62 N which is a ridiculously small force and certainly would not have the effect of pushing anyone in spaceship back against their seats.

The above arrived at value 6.23e-14 m/s^2 multiplied by the 2000 kg of the rocket gives the amount of thrust or force the rocket would have to generate in order to "hold station" at this distance and not slowly begin to drift towards the planet (1.25e-10 N).
A 80 kg man standing on the floor of said rocket would feel a force of ~5e-12 N Small, yes, but no more or less than what he would feel if standing on the surface of a planet with a mass equal to that of the Earth and 50 billion miles in radius.  It is also exactly the same force he would feel pressing him against the floor if the rocket were accelerating at 6.23e10-14 m/s^2 in empty space with no gravitational forces present.  The point of the equivalence principle is that our passenger, would not be able to distinguish between "hovering" 50 billion miles from an Earth massed planet and the rocket accelerating at 6.23e10-14 m/s^2 through free space with there being no gravity.  The fact that the force he feels is in all practical effect too small to be measured doesn't mean that it is the same in both cases.

Where am I going with this? I am trying to show that the equivalence principle of Einstein’s has absolutely no real meaning, it is just using gravitation as an explanation of itself. Here we have an object in space being accelerated to an equivalent acceleration of the acceleration due to the gravity on earth (i.e., 9.8 m/s2 ) yet it has no effect, some gravitational mass is needed close by to make it work.

No. What you have shown here is that you don't actually grasp the meaning behind the equivalence principle. The flaw is in your understanding, not the principle itself.

*more accurate would have been 8/5 x 10^3. And if you are going to rough out this by this degree (2 instead of 1.6, or only to to 1 significant digit), it really doesn't make sense to carry out the Earth's mass to 5 significant digits.

29

Marine Science / Re: How do tides work?

« on: 15/10/2019 16:28:42 »

Tides are the result of the difference in gravitational pull across the Earth caused by the Sun and Moon.
So for example, the Moon pulls on the near side of the Earth more than it pulls on its center, and pulls on the center harder than it pulls on the far side.  Also, since the Moon's gravity pulls towards the center of the Moon, it also tends to slightly squeeze in on the sides of the Earth. This results in the two tidal bulges we get.  So that when you have a high tide on one side of the Earth you will have a high tide on the other side. ( this is why we get 2 high tides and 2 low tides in roughly 24 hrs.)
The Earth is tilted with respect to the both its orbit around the Sun and the Moon's orbit around it.  This results is the tidal bulge peaks to be at latitudes away from the equator most of the time.  Sometimes, like during full and new Moons, the Sun Earth and Moon are more or less in a line and the pulls of the Sun and Moon align.  This results in their tidal effects adding up and we get higher than average high tides and lower than average low tides, known as Spring tides.  During quarter Moons, the tidal effects tend to cancel each other out and we get the least variation between low and high tides, these are known as Neap tides.
Inland lakes can have tides, but the lake has to be fairly large for them to be measurable. Lake Superior has a tide, but it's Spring tides are only 5 cm in height.

Another effect is that open ocean tides are only about 1 meter in height.  The much larger tides we get at the coasts of continents is due to a "sloshing" effect.   If you are on an Island in the tropics, you will only see the open ocean tidal difference.

30

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 15/10/2019 01:38:04 »

Well, I just don't understand it... If it is infinite then why all the to-do and palava about "We've calculated the mass of the entire universe and it's been found wanting, and in need of a shed-load more mass, which just happens to be invisible" (Dark Matter). It's this statement that inspired me to question what CMBR actually is and why somebody thought it suggested there wasn't enough of it (mass that is).

Is dark-matter another myth that's been blown out of proportion?

We have looked at the observable universe how it behaves.  It behaves as if it contains a lot more mass than the mass we can see or detect via the electromagnetic spectrum.  So we have calculated the mass of the entire "observable universe", and found that visible mass only makes up a part of it.
On smaller scales, we look at galaxies. We can measure how fast the stars in them orbit.  But if we calculate how fast they they should orbit based on the matter we can see, they are orbiting too fast.  Not only that, but the way their orbital speeds change as you move out from the center of the galaxy isn't consistent with the way the matter we see is distributed. So for example with a typical spiral galaxy, instead of behaving like the matter is constrained to the visible disk shape, they behave as if the majority of the matter is spread out in a sphere which extends above and below the disk of the galaxy.
There are two possible explanations for this: Either there is something with mass there that we can't see, or our understanding of gravity is wrong.   Both possibilities have been examined and considered. But as more and more data comes in, the dark matter keeps gaining evidence and the other model loses it. For example, we've recently found some galaxies that behave as if they have little to no dark matter.  Now while is is perfectly reasonable for this to occur in the dark matter model, it is less reasonable that gravity itself behave differently in these galaxies then other.  If the rules governing gravity are different than those we think they are, they would still need to be consistent from galaxy to galaxy.
And It's not as if we don't already know of particles that behave similar to the way dark matter would. The neutrino is a massive particle, with gravity, which does not interact via the electromagnetic interaction. It is effectively invisible.  It is a WIMP (Weakly Interacting Massive Particle)*, which is one of the possible candidates for dark matter, In fact, one hypothesized type of neutrino( the Sterile Neutrino) has been suggested as a suspect.

*The other candidate is MACHOs or MAssive Compact Halo Objects

31

Physics, Astronomy & Cosmology / Re: Is light an independent entity, or is it an effect of a larger mechanism ?

« on: 14/10/2019 18:13:16 »

Is this a good time to ask about something I came across recently in a popular science video somewhere....

Concerning a particular type of particle which, when its 'state' was changed then a matching particle's state also changed - instantaneously, no matter how widely seperated in distance (thus appearing to break the lightspeed limit).

Sorry I forget the details and the video, because I didn't pay too much attention to it. But just wondering if this rings any bells here?

Like as already been alluded to such "quantum entanglement" can't be used to send actual information faster than c (speed of light in a vacuum.)  Using the glove example.  In QM, until you open an envelope, not only do you not know whether it contains a LH or RH glove, but neither does the "glove" know* which it is. It is in some undetermined state.  When you open your envelope and your glove "decides" what it is, the other entangled glove "decides" its the other.
The problem is that when you open your envelope to ans see a LH glove, you don't know if that is because you opened the envelope and set off the decision making process, or if someone else had already opened the other envelope, and that event decided what glove you would see. It would take some slower than light communication between you and where the other envelope is to determine this.  The same goes with the changing of state (turning the glove inside out).  You don't know if the inside-out glove was the original unchanged state or the "changed" state without communicating with the other end by some slower than light means.

* This is not to say that you could actually do this with an actual pair of gloves, the glove here is just stand-in for a subatomic particle. One of the reasons quantum behavior seems so strange to us is that by the time you get to the macroscopic scales these effects "average out" to being immeasurable, and how objects appear to behave at this scale is what we accept as "natural".

32

Physics, Astronomy & Cosmology / Re: Do half lives of radioactive materials affected by gravity potential?

« on: 09/10/2019 23:00:29 »

Two samples of identical radioactive material are placed separately, one on earth surface at sea level, while the other are in a satellite orbiting the earth. Will they have different half lives ?

Well no.  If some element has a half life of a million years, it will still be a million years anywhere.
But if I measure that million years using a clock that's not present with the element decaying, the dilation between the clock and the element being measured will not be the same, so the result can vary one way or another.

Yes, time dilation effects everything.  Below 3200 km, the orbital speed is high enough that time would run slower.  The ISS is a case of this.  Above that altitude, the speed is lower and the gravitational potential is higher, so time runs faster.  GPS satellites are an example of this.

Let's say the half life of the sample when measured on earth surface is 1 year. Let one sample orbits the earth for 10 years, and then sent back to earth. Will those samples have the same mass?

It depends on the altitude of the orbit.   A clock sitting on the surface of the Earth, as measured by a distant observer shows a time time dilation due to it gravitational potential relative to the Earth. For an orbiting clock, both the potential and orbital speed come into play.   
Now according to our far off observer, the higher orbiting clock run faster than the lower surface clock if they just consider gravity, but the orbital speed adds a factor which adds an additional amount of slowing.  Now the lower the orbit, the less the difference in gravitational potential, but the higher the orbital speed.
So for example, it the orbiting clock were able to orbit the Earth at the same height as the ground clock, then there would be no difference in tick rate due to gravitational potential, any time dilation difference would be just due to orbital velocity, and the orbiting clock would be ticking slower.
However, if you move the orbiting clock to some really high altitude ( say moon orbit distance), the gravitational potential difference grows and the orbital speed goes down. Now the orbiting clock is runs fast compared to the ground clock by an amount that is greater than what it is slowed by orbital speed.  The orbiting clock runs fast compared to the ground clock.
Somewhere between these two cases is an orbit where the two effects cancel each other out and the orbiting clock and ground clock tick at the same rate.
If R is the radius of the Earth, then r the radius of the orbit  when this occurs is found by
r = 3R/2
Ergo, if the orbiting clock orbits at an altitude 1/2 an Earth radius above the ground, it ticks at the same rate as the ground clock.  Orbiting lower means it ticks slower than the ground clock and orbiting higher means it ticks faster.

So for example, a GPS clock would be expected to run slow by ~38 microseconds per day.  Over ten years, that would accumulate to a difference of ~0.13 sec.  So an isotope with a half-life of one year would decay imperceptibly less in orbit than on the Earth.  Since radioactive decay is statistical in nature, the difference may even be within that statistical variation. (in other words, two samples side by side may vary in result by more than the difference expected from time dilation in this example for any reasonable size of sample.)

33

The Environment / Re: Why do clouds stay high up when they are heavier than air?

« on: 08/10/2019 17:47:50 »

Is a cloud gaseous water or condensed water?

yes


The boundary of the cloud is not a true edge: water is going in and out of the cloud all the time. But only within the region of the cloud is the aerosol stable. Any water outside of the cloud is either in the gaseous state, or is precipitations (solid or liquid).

A good example of this is the lenticular cloud that can form on the leeward side of a mountain.  The wind pushes humid air up the windward slope which condenses on the leeward side.  The cloud formed remains in position with respect to the mountain, but the moisture forming it is constantly being removed by the wind, as the same wind brings in new moisture.

34

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 08/10/2019 17:28:34 »

And just to add to What Kryptid already said. While the photons we detect today are different ones than the ones we detected yesterday or will detect tomorrow. The one we detected yesterday where a bit "younger" and and the one we will detect tomorrow will be a bit "older".   In other words, the CMB is still decreasing in temp as the universe continues to expand.  The decrease is very small, so it would take a long time before it became enough to be discernible by our measurement standards, but it exists.


 

So the radiation we saw last week must now be gone forever. It sounds like this radiation must have been tavelling past us and 'disappearing forever' for the last umpteen billion years. why is it STIIL travelling past us in a way that we can register with instruments?

This is like saying that because the average water molecule speed is something is in the 100's of meters per second, that a fish in a lake should not detect any water around him because the water molecules that he felt yesterday has traveled off to some other part of the Lake. New water molecules have replaced them.*

*Now it is true that with the lake being a confined body of fixed size, he can encounter the same water molecule more than once.  This is note completely inconceivable for the universe if it were of a finite fixed size.  a Photon could pass the Earth, "circumnavigate" the universe and pass it again. How long it takes between passes would depend on the size of the universe. ( in a really small "closed" universe you could conceivably look off into the distance with a telescope and see the back of your own head)
 But with an expanding universe, even if it is finite, the size and rate of expansion could be such that the "circumference" grows faster than light can traverse it, and light passing you at any given moment can never return. ( but this still doesn't mean that it won't be replaced by a different photon.)

35

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 08/10/2019 16:02:23 »

The problem is that you are thinking of the Big Bang as an explosion in space, rather than the expansion of space.
With an explosion in space, the Photons could have raced past the matter into the space beyond. But with the Big Bang, there is no "space beyond" for the photon to expand into. We are at the "forefront" of this expansion.
An analogy would be that the universe is like the skin of an expanding soap bubble. Everything in it, photons, matter, etc. is confined to the skin. When the bubble is small everything is much more crowded together and as it expands, Things on a whole move apart and it is less crowded.  With photons, when it is small their frequencies are high ( the universe is "hot"), but as the bubble expands, they are stretched out to longer wavelengths and lower frequencies ( the universe "cools"). The CMB are those early photons that have been stretched out, but still confined to the same "Bubble skin" as everything else.

36

Physics, Astronomy & Cosmology / Re: Why are my atoms not flying apart? Please Hurry - May Have Limited Time Left

« on: 21/09/2019 18:14:23 »

Okay, I'm not really worried about disintegrating.  Just wanted to get your attention.  I asked the following question on a more general forum and got a single, not-so-satisfactory answer.  So here goes:

If the very fabric of space is expanding, then I would assume that such expansion is resulting in increasing distances between all physical objects -- not only macroscopic objects like galaxies, but also between the very particles that make up atoms.  I get that at that scale, such increases in distance would be infinitesimally minuscule -- but I also assume not zero.

My layperson understanding of particle physics tells me that the strength of the forces governing subatomic interactions are very critical -- and measured to many, many decimals of precision.  Furthermore, if the constants were reduced by the even the slightest amount, one result would be that nucleonic particles would not be able to adhere (or whatever the term of art is) to form stable nuclei.

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

As of this time, Atomic radii are not increasing due to the expansion of the universe.  The bonds holding them togehter are stronger than the effect caused by the expansion. 
To explain, I'll use a somewhat loose analogy.   Imagine you and someone else are standing on a waxed tile floor in your stocking feet,  each standing in the center of am adjacent tile.  The tiles all begin to expand.  You and your friend will move apart. But if you grip hands, the floor will still expand but you won't move apart.  The strength of your grip is stronger than the friction between your socks and floor.  This is somewhat similar to what is happening with the universe expansion. The expansion, if it could, would separate adjoining atoms, but the force holding the atom together holds them in place.
This is true not only for atoms, but galaxies and groups of galaxies.  The gravitational bound is stronger than the effect of the expansion and holds them together against it and keeps them from expanding along with the universe.  Galaxies and groups of galaxies retain their size, while moving apart from other galaxy groups.

Now, having said that, this may not always be the case.  A while back we learned that the expansion of the universe is accelerating and the that the rate at which it expands is increasing.
So let's go back to the tiled floor.  If we keep increasing the rate at which the floor expands, the tendency for the soles of your feet being dragged along increases.  Eventually, it will exceed the strength of your grip and you will be pulled apart.
In cosmology, this eventual result due to an ever increasing rate of expansion is call the "Big Rip".
As the rate continues to increase, the ability for structures to hold together against it begins to fail.  First groups of galaxies will begin to pull apart,  then later galaxies themselves will pull apart into independent star systems. Down the road, the planets of those star systems will be ripped away.   Going further forward more, the planets and stars will be ripped apart as their gravity will no longer be enough to hold them together.  This will continue as smaller and smaller structures succumb.

If this turns out to be the eventual fate of the universe, it turns out that the end comes fairly quickly. For one model, which has the end coming in 22 billion years, galaxies would be pulled apart ~60 million years before the end, and our solar system would hold together until just three months until the end, planets and stars wouldn't go until minutes before the end, and atoms just moments prior to the end.

And we really don't know if the the Big Rip scenario is the actual fate of the universe.

37

Chemistry / Re: Where does the spent car tire rubber go?

« on: 06/09/2019 17:47:46 »

One thing to keep in mind is that the rubber will decompose over time.   A car tire will completely decompose in 50-80 yrs.
But this doesn't mean that there should still be 50-80 yrs worth of accumulation of worn tire rubber left around.  That 50-80 yr figure is for a tire that is still one large single lump of rubber not already broken up into innumerable small particles.
An analogy is like dropping a salt lick block whole into a lake and grinding it to a powder first and scattering it over the whole lake. The block will take a lot longer to dissolve than the powder will.

Stupid question but micro plastics hang around and never completely disappear. They apparently wind up back in the food chain. Does rubber and plastic not decompose in a similar way, do we have micro rubbers:) in the food chain as well as plastics.

No, rubber and plastic do not decompose the same way, not even all plastics decompose the same.  As stated, an automobile tire will decompose entirely in 50-80 years. A plastic bottle will take ~450 yrs to decompose. Styrofoam doesn't decompose at all.  So it is Styrofoam and like materials that produce the "everlasting" micro-plastics, not all plastics.  That is not to say you can't have micro-particles of rubber or plastics in the food chain, especially if we are producing the materials at a faster rate than they decompose. 

38

Physics, Astronomy & Cosmology / Re: How Can This Black Hole Be So Big Ton 618 ?

« on: 21/08/2019 16:33:12 »

See: https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation

If black holes evaporate via Hawking radiation, a supermassive black hole with a mass of 1011 (100 billion) M☉ will evaporate in around 2×10^100 years . Would galaxies drift apart without their blackholes??

Estimated mass of SMBH at center of our galaxy: 4 million solar masses.
Total estimated mass of our galaxy 700 billion solar masses.
So the SMBH only makes up 0.0006% of the mass of the galaxy.  It's loss would have an insignificant effect on the dynamics of the galaxy.
For comparison the Sun converts about the same percentage of its mass to light in just under 1,000,000 years.  Ergo, A galaxy would loose mass via radiation loss at a faster rate than it would by SMBH evaporation, even if the SMBH were free to evaporate. 
Secondly, For a BH to even lose net mass via evaporation, It would need to lose energy/mass at a faster rate than it takes in.  Matter, starlight, and even the Cosmic background radiation all all sources which will feed the BH. 
The basic rule is that the Hawking temp of the BH has to higher than the average ambient temp of its surroundings.  Even if we remove all other sources, the CMBR still remains.  Hawking Temp goes down with size, and beyond a certain size, a BH will be below the CMBR temp.    This mass is approximately equal to that of our moon.

Ergo, before any SMBH at the center of any galaxy can even start to experience a net loss of mass via Hawking radiation, the universe will have to have expanded and cooled by a great deal, the galaxies would have had to used up their source of hydrogen for forming new stars, and remaining husks of old stars will have to have cooled significantly.( considering the fact that small red dwarfs can burn for trillions of years, and It can take a quadrillion years (or longer) for the white dwarf formed at the end of solar mass range stars to cool to black dwarf stage, you are going to be for a long wait.   If the "Big Rip" hypothesis is correct, run away expansion of the universe would have torn galaxies, star systems and even stars themselves apart before then, so it would be a bit of a moot point.

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Physics, Astronomy & Cosmology / Re: Where were the electrons before the big bang?

« on: 22/06/2019 15:54:14 »

During the earliest stages of the universe, the fundamental forces were unified into a single force( or at least were all of the same strength).  This means that they did not even exist in forms that they have today.  As the universe cooled, different forces separated out from that mix, one by one. First gravity, then the strong force...   Thus when the "quark soup" was in sway, the electromagnetic force had yet to come into existence as a singular force, so the entity we now call the electron wasn't possible.
To use an analogy, its like when you freeze water.   It doesn't really make sense to ask what "form" or shape the crystal structure of ice took before the water started to freeze, as the crystals depend on the water having the properties to allow them to form.

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Physics, Astronomy & Cosmology / Re: Do Peking and Helsinki lie on a straight line through Earth's core?

« on: 22/06/2019 01:16:02 »

The red line represents the straight-line path.  The yellow sphere represents the extent of the outer core.


core.png (122.53 kB . 960x540 - viewed 2165 times)

So the question is, What do you mean by "pass the core"?   Do you mean "pass through" the core, or do you mean "bypass" the core without entering it?  An if you mean bypass, do you mean it has to be close miss or does any miss count?  It is obvious that the line never enters the outer core, nor does it come very close.  so the answer to your question depends on exactly what the question is.